FR2677636A1 - Material based on manganese dioxide and electrochemical cell containing it - Google Patents

Abstract

The present invention relates to a material based on manganese dioxide, characterised in that it has a predominantly ramsdellite structure, it is highly crystalline, it is prepared chemically and it has a powder X-ray diffraction diagram (K line of Cu) in which the ratio of the height of a (110) peak to the height of a (201) peak is at least 0.6:1.0; it also relates to an electrochemical cell (10) which comprises: - an anode (12) - a conductive cathode (16) comprising the said material based on manganese dioxide - an electrically insulating electrolyte (14) separating the anode and the cathode.

Description

MANGANESE DIOXIDE MATERIAL AND CELL
ELECTROCHEMICAL CONTAINING THE SAME
The subject of this invention is a material based on manganese dioxide. It also relates to an electrochemical cell containing the material.

 The first object of the invention is a material based on manganese dioxide which is highly crystalline, chemically prepared, which has a predominantly ramsdellite structure and which has an X-ray diffraction diagram of the powder (Ka line of Cu) in which the ratio of the height of a peak [110] to the height of a peak [201] is at least 0.6: 1.0.

 The material will be primarily used in electrochemical applications, typically as an electrode material in an electrochemical cell having an electrochemically conductive anode, an electrochemically conductive cathode and an electrochemically insulating electrolyte separating the anode from the cathode.

 The material can therefore be used as a positive electrode material, or cathode, in a cell comprising aqueous or non-aqueous electrolytes, for example those which employ a negative anode or electrode of zinc or lithium. It is believed that it will find particular, but not necessarily exclusive, application as a positive electrode material in primary or rechargeable lithium cells.

 The material may also comprise a minor proportion of MnO2 ss, namely MnO2 of interstitial rutile structure in combination with the predominantly ramsdellite structure.

The material may also include a minor proportion of lithium or hydrogen to stabilize the ramsdellite structure. In this regard, it is not necessary that the
MnO2 ramsdellite is a stoichiometric compound in which the Mn: O ratio is 1: 2, so that the oxidation state of the manganese ions is 4.0, but it can be a compound in which the Mn: O ratio deviates slightly from 1: 2, so that the oxidation state of the manganese ions is less than 4.0 but more than 3.5, preferably more than 3.8.

 In the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of the peak [201] can be at least 0.8: 1.0, typically about 1.0: 1.0, which indicates a high degree of crystallinity and a single-phase character of the structure of ramsdellite manganese dioxide. Peak [110] can be around 20 = 22, while peak [201] can be around 20 = 37. In the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of a peak [221], which can be around 2e = 560, can be at least 1.2 : 1.0, which is also an indication of the high degree of crystallinity of the structure of ramsdellite manganese dioxide. The ratio of the height of peak [110] to the height of peak [221] can be around 1, 4: 1.0. The peak [110] can have a peak width, at mid-height, less than 20 = 2, for example around 28 = 1.5, which is an additional indication of the high degree of crystallinity of the dioxide structure. of manganese ramsdellite.

The material can be prepared by reacting a lithium oxide and manganese compound with concentrated acid. The acid can be sulfuric acid and its concentration can be at least 2M. It has been found that using a concentrated acid to digest the lithium oxide and manganese compound results in the synthesis of a highly crystalline structure of MnO2 ramsdellite. It is believed that an advantage of the high degree of crystallinity in the ramsdellite structure or phase is that the integrity of the ramsdellite MnO2 structure during a cycle in rechargeable electrochemical cells of the type
Li / MnO2 ramsdellite can be better than that obtained from MnO2 γ known as
MnO2 prepared electrolytically ("DME"). Another advantage of the ramsdellite MnO2 structure is that it can offer a higher initial discharge capacity for applications to primary cells compared to known materials of chemically prepared MnO2 type ("DMC") and to DME products.

The precursor compound of lithium and manganese oxide can be chosen from stoichiometric spinel compounds, such as LiMn2O4, or defective spinel compounds such as those encountered in the system.
Li2O, yMnO2, for example Li2Mn4O9 (y = 4) or Li2Mn307 (y = 3).

These precursor compounds can typically be synthesized by reaction of manganese carbonate, MnCO3 and lithium carbonate, Li2CO3, in the required ratios and at predetermined temperatures, for example Li2CO3 + 4MnCO3 800 C 2LiMn2O4 + 5CO
air
# 400 C
Li2CO3 + 4MnCO3 air Li2Mn4O9 + SCO2
# 400 C
Li2CO3 + 3MnCO3 air Li2Mn3O7 + 4CO2
The lithium-manganese oxide spinel precursor compounds can be digested in concentrated sulfuric acid, for example H2SO4 2.6M, at high temperature, for example at around 95 ° C., for several hours, to effectively leach all of it. of lithium and generate the highly crystalline MnO2 ramsdellite phase. For example, when led to the end, ideal reactions can be represented by
2LiMn2O4 -> 3MnO2 + MnO + Li2O
Li2Mn4O9 -> 4MnO2 + Li2O
Li2Mn307 -> 3MnO2 + Li2O
It should be noted, however, that in practice the final ramsdellite phase may contain a small amount of lithium or hydrogen which is believed to be used to stabilize the structure.

 The MnO2 ramsdellite phase, when it is prepared by the process according to the invention, usually contains a small amount of water which is normally associated with the surface of the MnO2 particles or with the grain boundaries.

This water content is important when MnO2 ramsdellite must be used as an electrode in aqueous cells, for example those which employ zinc anodes. However, when used in lithium cells, the MnO2 ramsdellite phase should be heated to 100u or more to remove water from it. It has been found, in this regard, that the MnO2 ramsdellite phase of the present invention is remarkably stable at 250-300 C; however, a heat treatment above 300 C causes a transformation into an MnO2 ss (rutile) structure.

The MnO2 ramsdellite phase can also be dehydrated at high temperature, for example at 200-400 C, preferably at 300-370 C, in the presence of a lithium salt such as LiOH,
LiNO3 or Li2CO3, to generate lithium stabilized ramsdellite phases, possibly in the presence of additional lithium oxide and manganese phases, such as spinel phases, which can be produced as a reaction byproduct. Although the exact domain of the composition of these phases has not been determined, it is believed that the overall composition can be represented by LizxMnO2 + x, with o S xc 0.2. It will also be noted that the constituent MnOZ in the Li2xMfl02 + x phases related to ramsdellite do not need to be stoichiometric, but may have a slight oxygen deficit, so that the oxidation state of the manganese cations is slightly lower 4.0
Although this material has been described as being suitable for use as an electrode material, it is believed that it can also be used in catalytic applications.

 The second object of the invention relates to a material based on manganese dioxide which is highly crystalline, which has a predominantly ramsdellite structure and which has an X-ray diffraction diagram of the powder (Ka line of Cu) in which the ratio of the height of a peak [110] at the height of a peak [201] is at least 0.6: 1.0 and in which the peak [110] has a peak width, halfway up, less than 20 = 2.

 The material according to the second subject of the invention can also be prepared chemically as described above and can have peak heights and relative peak widths as described above.

The third object of the invention relates to an electrochemical cell, which comprises
an electronically conductive anode;
an electronically conductive cathode comprising a highly crystalline manganese dioxide, chemically prepared, which has a predominantly ramsdellite structure and which has an X-ray powder diffraction pattern (Ka line of Cu) in which the ratio of the height of a peak [110] at the height of a peak [201] is at least 0.6: 1.0; and
an electronically insulating electrolyte separating the anode from the cathode.

 The cell can be a primary or secondary cell, namely a rechargeable cell, and the electrolyte can be aqueous or non-aqueous, the anode then being, for example, zinc or hydrogen, in the case of an aqueous electrolyte, or lithium in the case of a nonaqueous electrolyte.

A fourth subject of the invention relates to an electrochemical cell, which comprises
an electronically conductive anode;
an electronically conductive cathode comprising a highly crystalline manganese dioxide which has a predominantly ramsdellite structure and which has an X-ray powder diffraction pattern (K a line of Cu) in which the ratio of the height of a peak [110] at the height of a peak [201] is at least 0.6: 1.0; and wherein the peak [110] has a peak width, at mid-height, less than 28 = 2; and
an electronically insulating electrolyte separating the anode from the cathode.

 Manganese dioxide can be as described above and can have, in particular, an X-ray diffraction pattern of the powder as described above.

 The invention will now be described with the aid of the following illustrative examples of electrode materials and cells according to the invention, and with the aid of the appended schematic drawing (FIG. 1) which is a schematic representation of a cell according to the invention.

 In the drawing, we see a Li (anode) / LiC104 1M type cell in propylene carbonate (electrolyte) / MnO2 ramsdellite, Teflon, acetylene black (cathode). The cell is designated by 10 and the anode, the electrolyte and the cathode are respectively designated by 12, 14 and 16. The anode, the electrolyte and the cathode are contained in an insulating housing 18, the anode being separated of the cathode by the electrolyte, and suitable terminals (not shown) are in electronic contact with the anode and the cathode respectively.

 In the cathode, Teflon is a binder, acetylene black is a current collector. The MnO2 ramsdellite, in powder form, is mixed in a mass proportion of 70-80% of MnO2, with 30-20% of Teflon and acetymene black, Teflon and acetylene black being in a mass ratio of 1 : 2, and compacted at 5-10 MPa.

 The MnO2 ramsdellite which is suitable for use in the cathode 16 of cell 10 is prepared according to the examples below.

EXAMPLE 1
A stoichiometric spinel LiMn204 is prepared by reacting an intimate mixture of Li2CO3 and MnCO3, in a molar ratio of 1: 4, at 800 ° C. in air for 24 hours. Then the LiMn204 precursor product in 2.6M H 2 SO 4 is heated to reflux at 950C for 2 days. The X-ray diffraction diagram of the powder (Ka line of Cu) of the resulting MnO2 ramsdellite product is shown in Figure 2. After drying the product overnight at 100 C, the concentration [H +] is 0.16% by weight , which indicates a small amount of residual water on the surface and water occluded in the structure. After heating the ramsdellite product to 250 C, there is no significant variation in the X-ray diffraction pattern of the powder (Figure 3), which indicates the integrity of the ramsdellite phase structure at this temperature. The hydrogen content [H +] of the ramsdellite product heated to 250 C is 0.08% by weight.

The high degree of crystallinity of the MnO2 ramsdellite phase of the compound of this invention is reflected by the relatively sharp peaks and, in particular, by the sharp and intense peak [110] at about 28 = 22 ", which has a peak height relative about 1.0: 1.0, relative to peak [201] to about 28 = 37 #, and a relative peak height of about 1.4: 1.0, relative to peak [221] at around 28 = 56 #. The ratios of the heights of the peaks [110]: [201] and [110]: [221] should therefore be, for a highly crystalline MnO2 ramsdellite according to the invention, preferably respectively greater than 0, 6: 1.0 and 1.2: 1.0, as noted above. In addition, peak [110] has a peak width at half height less than 28 = 2, which is another indication of the high degree of crystallinity of the
MnO2 ramsdellite of the compound of the invention. The structure of the MnO2 ramsdellite phase, as determined from a refinement of the profile of this diffraction diagram
X, is shown in Figure 4. The refinement shows that about 10% of the manganese ions, designated by o in Figure 4, are located in the channels (2x1). This characteristic can also be attributed to a small amount of interstitial MnO2 ss in the structure.

 The insertion of lithium into the ramsdellite phase is demonstrated by the reaction of a mole equivalent of nbutyllithium in hexane with the MnO2 ramsdellite at 45 ° C. for 4 days. The X-ray diffraction diagram of the powder of the lithiated product of composition Li0.5MnO2 is shown in FIG. 5. The appearance of several new peaks and the significant displacement of certain peaks, for example the peak [110] at a value of about 20 = 22 to about 20 = 19.5 is an indication of a modified ramsdellite structure and an expansion of the unit cell. The conservation of sharp, well resolved peaks, for example peaks [110] and [201] , indicates that the lithiated phase retains a high degree of crystallinity, even after a reaction with a strong reducing agent such as n-butyllithium. The modified ramsdellite structure, as determined by a refinement of the profile of the X-ray diffraction diagram, is shown in FIG. 6. It shows that the insertion of lithium is accompanied by warping of the oxygen planes and shearing of these planes, to pass from a compact hexagonal stack to a compact cubic stack structure.

 A crystallographic analysis of the product in FIG. 2 by profile analysis of the X-ray diffraction diagram indicates that the product is an almost pure ramsdellite phase having an orthorhombic unit cell with lattice constants a = 9.376 A, b = 4.471 A and c = 2.855 At, i.e. respectively a = O, 9376 pm, b = 0.44471 m and c = 0.2 # 55 #m. The determination made on the partially lithine phase Li015 # nO2, the X-ray diffraction diagram of which is shown in FIG. 5 gives lattice constants a = 9.527 A, b = 5.059 A and c-2.848 A, ie respectively a = O < 9527 pm, b = 0.5059 ym and c = t, 2848 pm.

EXAMPLE 2
A defective spinel Li2Mn4Og (Li2O, 4MnO2) is prepared by reacting an intimate mixture of Li2CO3 and powders.

of MnCO3, in a molar ratio of @: 4, in air at 400 C for 20 hours. Then, the precursor Li2Mn4O9 in 2.6M H 2 SO 4 is heated to reflux at 95 ° C. for 2 days. The X-ray diffraction diagram of the powder of the resulting product is shown in Figure 7; it is very similar to the product of Example 1 (Figure 2).

EXAMPLE 3
We react MnO2 ramsdellite in air with
LiNO3, at 280 C for 30 hours, then at 300 C for 20 hours. The Li: Mn ratio of the starting mixture is 3: 7. The X-ray diffraction pattern of the product powder is shown in Figure 8. The major peaks of the X-ray diffraction pattern can be attributed to a product
Li2xMnO2 + x, whose unit cell orthorhombic is such that a = 9,268 Â, b = 4,971 Â and c = 2,864 A.

EXAMPLE 4
The product of Example 1, heated at 100 ° C. overnight, to remove water, is evaluated as cathode material in lithium cells, analogous to cell 10 in FIG. 1. The cells consist of a metallic lithium anode 12, compacted on a stainless steel current collector, an electrolyte 14 comprising 1M LiC104 dissolved in propylene carbonate and dimethoxyethane, in a ratio of 1: 1 by volume, and a cathode 16 containing about 40 mg of MnO2 mixed with about 10 mg of a mixture of Teflon binder and acetylene black, in which Teflon acts as a binding agent and acetylene black as a current collector; the Teflon / acetylene black ratio of these mixtures is 1: 2.

 The initial discharge curves of the three independent lithium cells according to the invention are indicated in FIG. 9, which thus highlights the fact that the ramsdellite phase plays the role of effective cathode material, giving, on average, a capacity about 225 mAh / g on the initial discharge, up to a cut-off voltage of 2V. The open circuit voltage relative to the composition x, in LixMnO2, shows that the MnO2 ramsdellite can receive a Li up to a cut-off voltage of 2.8V, as shown in Figure 10.

 A cyclic voltampere graph of the MnO2 ramsdellite scanning the voltage range from 1.1 V to 4.6 V at a speed of 1 mV / s (Figure 11) demonstrates that the electrochemical reaction is reversible after the initial discharge cycle.

 The electrochemical discharge curves of the first 8 cycles of a rechargeable ramsdellite Li / MnO2 cell are shown in FIG. 12, which confirms the cyclic voltammeter results and the fact that, after the initial discharge, the cell gives a rechargeable capacity of between 100 and 150 mAh / g.

EXAMPLE 5
The MnO2 ramsdellite phase prepared by the method of Example 1, but not heat treated, is evaluated in an alkaline half-cell with a nickel grid counter electrode (anode), a KOH 9M electrolyte and a reference to Hg / HgO. The cathode consists of 500 mg of MnO2 ramsdellite mixed with 100 mg of graphite. The half-cell is discharged at a current intensity of approximately 10 mA (Figure 13). The voltage obtained from this electrode is satisfactory, while the theoretical discharge capacity (308 mAh / g) can be obtained by discharge at −1 V relative to Hg / HgO, which corresponds to the formation of MnOOH.

 A particular advantage of the invention is that it offers a lithium cell potentially suitable for primary or rechargeable use, simple design, low cost and good shelf life.

Manganese dioxide is a cathode material well known for an electrochemical cell employing zinc or lithium anodes, provided with an electronically insulating electrolyte separating the anode from the cathode. The most commonly used form of manganese dioxide is by far
MnO2 & which can be prepared chemically, namely chemical manganese dioxide ("DMC"), or electrolytically, namely electrolytic manganese dioxide ("DME"). MnO2 & (Figure 14) has a structure that can be considered as an intermediary between a rutile MnO2 structure (MnO2 ss) (Figure 15) and a ramsdellite MnO2 structure (Figure 16). Both DMC and DME contain both surface water and occluded water, which aid in the electrochemical discharge reaction when used as cathodes in aqueous zinc cell systems. However, this surface and occluded water, which is believed to be located mainly at the grain boundaries, should be removed from the manganese dioxide electrode material if used in lithium cells, since lithium reacts vigorously with water.A heat treatment of MnO2 & at 350-450 C, which removes about 80% of the water, but not all of the water, also causes a transformation of the structure in that one calls a phase of MnO26 / ss, namely a phase in which the rutile component (or MnO2 ss) in the structure is more important.

 Rutile MnO2 contains one-dimensional channels having a cross section defined by the size of an MnO6 octahedron; channels are thus defined as being channels (lxl). The ramsdellite channels are also one-dimensional, but the cross section of each channel is defined by two octahedra of MnO6 in one direction and by a single octahedron of MnO6 in an orthogonal direction, so the channels can be defined as channels (2x1).

 The electrochemical reactions of lithium batteries which use transition metal oxides or chalcogenur # cathodes are most often done by insertion or topochemical reactions, in which lithium is inserted in the oxide host structure / transition metal chalcogenide, which is accompanied by a reduction in the transition metal of the host.

Therefore, MnO2 ss with narrow unidimensional channels is not as electrochemically active as NnO2 &gamma; which contains both type channels
MnO2 ss (lxl) and larger ramsdellite type channels (2x1). It has been found that a crystalline product MnO2 p only absorbs 0.2 Li + per MnO2 motif, while MnOX &gamma; / ss heat treated, comprising both ramsdellite channels and rutile channels, accepts significantly more Li + ions per unit of formula. In particular, it has been found that the heat treated MnO2 # / # reacts with Li + by pattern of MnOv, but this phenomenon is not completely reversible, which limits the application in rechargeable lithium cells.

 Thus, the absorption capacity of lithium in the electrode material, and therefore the capacity for re-charging the electrode material, is all the greater as the fraction of ramsdellite in MnO2 &gamma; is higher.

 Since the electrode material of the present invention can be synthesized in a substantially anhydrous form, it is not necessary to have the relatively high temperatures which are necessary to remove moisture and which transform part of the structure into phase .w.nO2 p undesirable, as noted above. In addition, the desired stability of the ramsdellite structure can be obtained by reacting a lithium salt in minor concentration with the MnO2 ramsdellite, as indicated above.

 An X-ray diffraction diagram of the simulated powder of an ideal MnOZ ramsdellite structure is shown in Figure 17.

 The n03 ramsdellite has a distorted compact hexagonal oxygen ("hc") anion network. In such a disposition, the octahedra defined by the oxygen network share their edges with each other, while others share their faces. In the MnO2 ramsdellite, it is therefore unlikely that at any given time all the interstitial octahedral sites of the structure could be filled with inserted lithium ions, because of the electrostatic interactions of the cations in the octahedra with pooled faces. It is therefore believed that, in MnO2 ramsdellite, only a small fraction of the interstitial sites can fill with lithium ions before the oxygen ion network transforms into a compact cubic stack to give a modified structure which is inherently , more stable than the initial deformed compact hexagonal mother structure.

 It has also been demonstrated that, after the first discharge, all the lithium ions cannot be easily removed from the structure during the charging of the cell, and that a minor concentration of Li + ions remains in the cells. Channels of the ramsdellite phase modified to stabilize the structure.

Claims (18)

1. Material based on manganese dioxide, characterized in that it has a predominantly ranlsdellite structure, that it is highly crystalline, that it is chemically prepared and that it has an X-ray diffraction diagram of the powder ( Ka line of Cu> in which the ratio of the height of a peak [110) to the height of a peak t2013 is at least 0.6: 1.0. 2. Material according to claim 1, characterized in that it comprises a minor proportion of interstitial ss MnO2 in combination with the predominantly ramsdellite structure. 3. Material according to claim 1 or 2, characterized in that it comprises a minor proportion of lithium or hydrogen to stabilize the ramsdellite structure, the Mn: O ratio of MnO2 ramsdellite deviating slightly from 1: 2, so that the oxidation state of manganese ions is less than 4.0, but more than 3.5. 4. Material according to any one of claims 1 to 3, characterized in that, in the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of the peak [201] is at least 0.8: 1.0. 5. Material according to claim 4, characterized in that, in the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of the peak [201] is approximately 1.0: 1 , 0. 6. Material according to any one of claims 1 to 5, characterized in that, in the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of a peak [221] is at least 1.2: 1.0, which indicates a high degree of crystallinity of the ramsdellite manganese dioxide structure. 7. Material according to claim 6, characterized in that, in the X-ray diffraction diagram of the powder, the ratio of the height of the peak [110] to the height of the peak [221] is approximately 1.4: 1 , 0. 8. Material according to any one of claims 1 to 7, characterized in that, in the X-ray diffraction diagram of the powder, the peak [110] has a width of peak at half height less than 28 = 2 ', which is an additional indication of the high degree of crystallinity of the ramsdellite manganese dioxide structure. 9. Material according to claim 8, characterized in that, in the X-ray diffraction diagram of the powder, the peak [110] has a peak width at mid-height of approximately 20 = 1.5 10. Material according to l any of claims 1 to 9, characterized in that, when it is reacted with a lithium salt, it generates stabilized lithium phases which can be collectively represented by Li2 # MnO2 + #, or OS xc 0.2. 11. Material based on manganese dioxide, characterized in that it has a predominantly ramsdellite structure, that it is highly crystalline and that it has an X-ray diffraction diagram of the powder (Ka line of Cu) in which the ratio of the height of a peak 1110i to the height of a peak [20l] is at least 0.6: 1.0 and in which the peak [110] has a width & half-height less than 2e = 2 '. 12. Electrochemical cell, characterized in that it comprises  an electronically conductive anode;  an electronically conductive cathode comprising a highly crystalline manganese dioxide, chemically prepared, which has a predominantly ramsdellite structure and which has an X-ray diffraction diagram of the powder (Ka line of Cu) in which the ratio of the height of a peak [110) at the height of a peak [201] is at least 0.6: 1.0; and  an electronically insulating electrolyte separating the anode from the cathode. 13. electrochemical cell according to claim 12, ca characterized in that the cathode also comprises a propor 14. electrochemical cell according to claim 12 or 13, characterized in that the cathode also comprises a minor proportion of lithium or hydrogen to stabilize the ramsdellite structure, the Mn: O ratio of the MnO2 ramsdellite deviating slightly from 1: 2, so that the oxidation state of the manganese ions is less than 4.0, but greater than 3.5. 15. Electrochemical cell according to any one of claims 12 to 14, characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the ratio of the height of the peak [110] to the height of the peak [ 201] is at least 0.8: 1.0.  Electrochemical cell according to claim 15, characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the ratio of the height of the peak [110] to the height of the peak [201] is approximately 1, 0: 1.0. 17. Electrochemical cell according to any one of claims 12 to 16, characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the ratio of the height of the peak [110] to the height of a peak [221] is at least 1.2: 1.0, which also indicates a high degree of crystallinity in the structure of ramsdellite manganese dioxide. 18. The electrochemical cell according to claim 17, ca characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the ratio of the height of the peak [110] to the height of the peak [221] is about 1.4: 1.0. 19. Electrochemical cell according to any one of claims 12 to 18, characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the peak [110] has a peak width at half height less than 20 = 2, which is an additional indication to 2 # = 2, which is an additional indication of the high degree of crystallinity of the ramsdellite manganese dioxide structure. 20. Electrochemical cell according to claim 19, characterized in that, in the X-ray diffraction diagram of the manganese dioxide powder, the peak [110] has a width of peak 8 mid-height of approximately 20 = 1.5 , 21. Electrochemical cell according to any one of claims 12 to 20, characterized in that the manganese dioxide reacted with a lithium salt generates phases stabilized with lithium which can collectively be represented by Li2xMnO2 + x, or O sxs 0.2.  22. Electrochemical cell, characterized in that it comprises  an electronically conductive anode;  an electronically conductive cathode comprising a highly crystalline manganese dioxide which has a predominantly ramsdellite structure and which has an X-ray diffraction pattern of the powder (Ka line of Cu? in which the ratio of the height of a peak 1110) to the height of a peak [2011 is at least 0.6: 1.0 and in which the peak [130) has a peak width at half height less than 28 = 2 '; and  an electronically insulating electrolyte separating the anode from the cathode.